I don't think this is so far fetched. Morgans is mostlikely long enough in the business to give a proper estimation of the situation.
And i don't think that it's just based in this sole incident, this presumption is morelikely based on what the worst generation accomplished in such a short time, while the "old" Yonkous are standing still and preparing to defend their positions instead of taking the risk to make the next step.
You just can't become number one in a race, when you're just focused on defending your second place while looking behind. But if you look infront and take the risks on the third place you can overtake the second place to become number one.
The Worst Generation accomplished nothing worth of praise, aside from two or three of them. (again, i'm excluding Blackbeard who is a special case)
Kidd, Apoo, Hawkins, XDrake, Killer, were either utterly crushed and/or forced to serve under a Yonko. Yeah they got balls, but in the end they did nothing.
Jewery Bonney didn't do anything at all i think. Zoro either, aside from beating Pica...
Urouge defeated a Yonko commander. That is a good feat, but still he was crushed afterwards and forced to flee.
Luffy and Law defeated Doflamingo, a good feat but considered child plays for people of Yonko caliber.
Luffy and Bege attacked Big Mom, didn't do any damage, and were forced to flee to survive.
Like, at the end of the day what did the Worst Generation accomplished exactly. Yes they have balls to actually dare attack Yonkos. But each time they either got crushed, forced to serve under the Yonko they attacked, or were forced to escape for their lives.
I'm expecting Zoro and Law to have their time to shine soon, but for now, only Luffy is worth praising for beating 2 Yonko commanders (though he was helped by Nami against Cracker). And Urouge, who defeated Snack. All the rest of the Worst Generation didn't do crap.
Basically i can summarize it this way: if you take off Luffy from the Supernovas, would you still believe Morgan's claim that one of them will be Pirate King? I sure as hell would not considering what each of them did so far.